Variable filament inductor



Filed Sept. 27, 1951 C J STARNER VARIABLE FILAMENT INDUCTOR Nov. 25,1952 Patented Nov. 25, 1952 VARIABLE FILAMENT INDUCTOR Charles JamesStarner, Haddonfield, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application September 27, 1951, Serial No.248,501

11 Claims. 1

This invention relates to variable inductors, and more particularly to avariable inductor arrangement suitable for use in the filament circuitof a high-power electron discharge device capable of operating over alarge frequency range.

Triode amplifiers of the grounded-grid type may be utilized inhigh-power radio transmitters, particularly when the transmitters are tobe operated over a large frequency range. This type of operationrequires that the input voltage to the amplifier be appliedbetween.cathode and ground. If the amplifier tube is the direct heatingfilament type, the input ihductance is placed in series with thefilament power supply leads. In tuning over a large frequency range, itis necessary to vary the inductance. The inductance is usually varied byshorting out a portion of the inductor. However, the variation of theinductance produces a change in the voltage at the filament terminals ofthe tube due to the alteration of the filament circuit impedance at thefilament power frequency. In very high power amplifying tubes, thisvariation may exceed the manufacturers ratings for proper operation ofthe tube.

An object of the present invention, therefore, is to provide means forvarying the effective inductance of coils in series with the filamentleads of a grounded-grid triode amplifier without changing the filamentvoltage at the filament terminals of the electron discharge device.

Another object of this invention is to provide in the filament circuitof an electric tube an adjustable inductor circuit for radio frequencieswhich maintains a constant resistance to the filament heating powersupplied therethrough.

A feature of the invention comprises a novel arrangement employing apush-button shorting device for simultaneously var'ying the inductanceof a pair of coils by the same amount.

The invention is hereinafter described, by way of example only, inconnection with a groundedgrid triode amplifier capable of operatingover a frequency range of from four to twenty-seven megacycles at a highpower input. The two filament terminals of the vacuum tube are connectedto a filament transformer by means of an inductor in series With eachone of the two filament leads. Each inductor is,tuned by means of avariable tap connected to one end thereof. A resistor is connected inseries with the variable tap of each inductor. These resistors arechosen to have a high impedance to the 60 cycle filament voltage but alow impedance to radio frequency energy, thereby efiectively confiningthe filament heating current to the entire inductor. The re sistors maytake the form of nichrome ribbons An advantage of the present inventionis that the resistors, in addition to unctioning as connecting leads,also operate to dampen undesirable resonances in the shorted portions ofthe in ductors.

Other objects, features and advantages will appear in the subsequentdetailed description which is accompanied by a drawing, wherein:

Fig. 1 is a schematic diagram illustrating the application of thevariable inductor arrangement of the invention to a grounded-grid triodeFig. 2 illustrates a side elevational view of the mechanical structureof one of the variable filament inductors shown in Fig. 1, including apush button tuning mechanism; and

Fig. 3 is a top sectional view of Fig. 2 along line 33 illustrating thetwo variable filament inductors and including the resistor ribbons.

Throughout the figures of the drawing the same reference numbers areemployed to designate the same components.

Referring to Fig. 1, there is shown a grcundedgrid triode amplifieremploying an evacuated electron discharge device o having an anode l2, agrid M, and a filament l6. The filament 16 is coupled to the secondarywinding of a transformer l8 by the filament leads 29. The primarywinding of transformer IB is connected to a suitable alternating currentpower source 22. In series with the filament leads 2a are identicalinductors 24. Each inductor 24 is tuned by a variable tap 26 connectedto one end thereof. Two resistors 28 are connected so that one resistoris in series with each of the taps 25. The resistors 28 are chosen tohave a high impedance to the alternating current filament voltage but arelatively low impedan ce to radio frequency energy when compared withthe impedance of the inductor 24, thereby each resistor 23 eifectivelyconfines the filament heating current to the entire inductor 24. Aby-pass capacitor 3=3 connects each junction of the isolating resistor28 to ground.

An input tuning capacitor 32 is coupled in parallel with the filamentinductors by means of two blocking capacitors S4, each connected to afilament lead at the tube side of the inductors,

with their remaining ends interconnected and attached to one side of theinput tuning capacitor 32 at the junction 36. The other terminal of theinput tuning capacitor 32 is grcunded. The grid I l of the electrondischarge device H] is connected to a suitable bias source, not shown,by

means of the radio frequency choke 38. The grid is grounded at radiofrequences by a capacitor 39.

The anode l2 is connected to the anode inductor 40 of the anode tankcircuit 3l. The anode tank circuit 4l includes the series inductor M),capacitor 42, and capacitor G4. Capacitor 44 is connected to ground andforms part of the resonant output circuit. An anode voltage source, notshown, is coupled to the junction of the inductor 40 and capacitor 42 bymeans of the radio frequency choke 43. An anode ammeter 46, to indicateanode current, is attached between a tap on the secondary of thefilament transformer 18 and ground. The amplifier is driven by anexciter, not shown, over lead 48 coupled to the junction 36. Theamplifier is con nected to an antenna tuner, not shown, over a lead 50connected to the junction of capacitors 42 and 44.

The input circuit between the ungrounded terminal of the input tuningcapacitor 32 and ground is thus composed of the parallel tunablearrangement including the input tuning capacitor 32 in one leg of thecircuit and the unshorted sections of the inductors 24, each in serieswith its bypass capacitor 3B, in parallel, as the other leg. Tuning ofthe input circuit is accomplished by selecting the proper tap on thevariable inductors 24 and varying the input tuning capacitor 32 untilresonance at the desired frequency is reached.

Referring to Figs. 2 and 3, since the structure and operation of eachvariable inductor is similar, only one will be described whereconvenient. Each inductor 24 is in the form of a helix with spaced turnsand is inserted in series with one of the filament leads by means of aconnector 52 and the connecting bus 54.

The resistor 28 functions as the connecting lead between a lowerterminal 56 on the bottom turn of the inductor 24 and a mouhting plateES. A contact bar 63 extends the length of each inductor 24. The contactbar 60 is positioned in relation to the remaining structure by means ofinsulators 62, and is electrically connected to the mounting plate 58 bymeans of the link conductors 64.

The value of inductance in the tuned input circuit is varied by shortingout sections of each inductor 24. This is accomplished by a pair ofshorting contacts 66 attached to the end of each depressable insulatedpush button 68, which is positioned to connect the contact bar 69 to apredetermined turn of the inductor 24 when the appropriate push buttonis depressed, This is best seen by an inspection of the top sectionalview of Fig. 3. Since the inductors 24 are identical and are mounted injuxtaposition to each other, depressing one push button B8 eiects aconnection of a corresponding turn on each inductor 24 with itsassociated contact bar 60 to simultaneously vary the inductors by thesame amount.

The bypass capacitor 30 (shown in this figure as consisting of aplurality of paralleled capacitors) is connected between the mountingplate 58 and the chassis El which is at ground potential. The variableinput tuning capacitor 32 is mounted on a supporting plate 12 which isinsulated from the chassis 10. One side of the variable input tuningcapacitor 32 is directly connected to the chassis through a ground busbar M. Each blocking capacitor 34 is mounted between the supportingplate 'l2 and each filament terminal bar 76 to which the filament leads20 are attached. The filament terminal bar 6 for each filament lead 2)is supported by an insulating support 78. The blocking capacitors 34 arethus in series with one side of the variable input tuning capacitor 32and the filament leads 20. The upper end of each inductor 24 isconnected to each filament terminal bar 16 by means of a connecting bus54.

The blocking capacitors 34 have a value which gives a low reactance atradio frequencies and a high reactance at the power line frequency (e.g. 60 cycles per second). The bypass capacitors are also of a valuewhich have a low reactance at radio frequencies and a high reactance atthe power line frequency.

The following is an example of the improvement in amplifier operatingconditions which result when the invention is utlized in an actualembodiment. An RCA type 9C25 triode was employed. The filament of thiselectron tube required approximately 280 amperes at 6 volts. Since thefilament current must be supplied through the filament inductors 24, aconsiderable voltage drop results when the entire length of each of theinductors is used. When the inductors are completely shorted by theshorting contacts 65, there is practically no voltage drop. The voltagedifierence with the inductors completely shorted or entirely in thecircuit is approximately 0.3 volt. Since this voltage drop is inaddition to other normal variations of voltage, the tube filamentvoltage can not be maintained within the manufacturers ratings.

Utilizing the system of the present invention, however, thesedifliculties may be overcome. Employing the circuit shown above in Fig.l, the resistors 28 are made in the form of a 6 inch length of nichromeribbon, inch wide and 20 mils thick. In the improved arrangement, theratio of resistance of each resistor 28 to the resistance of thecorresponding inductor 24 is in the order of 10:1. More exactly, eachresistor 28 had a value of .02 ohm and the resistance of each entireinductor 24 was .002 ohm, so that practically all of the 60 cyclefilament current is carried by the entire length of the inductor and thevariation in filament Voltage is reduced by a factor of more than ten.

Thus, in accordance with the invention, the inductors in series with thefilament supply can be tuned over a very wide range of frequencies andthe filament supply voltage still be kept within the range ofpermissible variation. To accomplish the same degree of improvementutilizing the conventional blocking capacitor type of arrangementinstead of the systemof this invention would require a plurality ofcapacitors, the cost of which would be more than 530 times the cost ofthe resistors in this system. An addi tional advantage is also realizedin the corresponding saving in space requirements and maintenanceexpense. Furthermore, the nsertion of the resistors as shown in thecircuit in Fig. 1 is useful as a damping device to prevent unwantedresonances in the shorted portions of the inductors 24, thus reducingharmonic generation. The resistors also substantially reduce the amountof Current that the shorting contacts 66 have to carry, sincepractically no filament heating current flows in the shorting circuit.

What is claimed is:

1. An electron discharge device system comprising an electron dischargedevice having a heating element, means supplying heating current to saidheating element, an inductor con nected between said heating element andsaid supply means, a variable tap connecting one point of said inductorto a second point on said inductor intermediate its ends, and a resistorof high resistance compared to the resistance of said inductor connectedin series with said tap between said first point and said second point.

2. In combination, an electron discharge device having a filament, anenergizing source having two terminals, inductors respectively connectinthe filament terminals to different terminals of said source, a variabletap connecting one end of each of the inductors to a point on the sameinductor intermediate its ends, and a resistor having a relatively highimpedance to energy from said source compared to the impedance of theinductor connected respectively between said end of each inductor andits corresponding tap.

3, Apparatus for varying inductance in series with a heating elementlead of an electron discharge device comprising an electron dischargedevice having a heating element, a heatin element energizing source,leads connectin the said source to the said heating element, an inductorin series with one of the leads, a variable tap con necting one terminalof the inductor to a point intermediate between its ends, a resistorwith a relatively high impedance to energy from said source incomparison with the impedance of the inductor connected in series withthe tap and said one terminal, and means for supplying radio frequencyenergy to the other terminal of said inductor.

4. Apparatus for varying inductance in series with a heating elementlead of an electron discharge device comprising an electron dischargedevice having a heating element, a heating element energizing source,leads connecting the said source to the said heating element, aninductor in series with one of the leads, a variable tap connecting oneterminal of the inductor to a point intermediate between its ends, aresistor with a relatively high impedance to energy from said source incomparison with the impedance of the inductor connected in series withthe tap and said one terminal, said inductor forming part of a paralleltuned cathode circuit for said device, said tuned cathode circuitincludin a capacitor arrangement in shunt to said inductor.

5. An electron discharge device system comprising an electron dischargedevice having a heating element, a source of heating current supply,connection means supplying heating current to said heating element fromsaid source, an inductor in said connection in series between saidheating element and said source, a variable tap connecting a terminal ofsaid inductor to a point on said inductor intermediate its ends, and aresistor of high resistance compared to the resistance of said inductorconnected in series with said tap between said terminal and said point.

6. Apparatus for varying inductance in series with a heating elementlead of an electron discharge device comprising a heatin element forsaid electron discharge device, a heating element energizing source, twoleads Connectihg the energizing source to said heating element, aninductor having two terminals connected in series with one of saidleads, a contact plate in juxtaposition to said inductor, an isolatingresistor with a relatively high impedance to the heating energy incomparison with the impedance of said inductor, said resistor connectedbetween one terminal of said inductor and said contact plate,

and a plurality of push buttons, each of said push buttons having ashorting contact attached to the end thereof adapted to makesimultaneous contact with the contact plate and a predetermined turn ofsaid inductor when the push button corresponding to said turn isdepressed.

7. Apparatus for varying inductance in series with a heating elementlead of an electron discharge device comprising a heating element forsaid electron discharge device, a heatin element energizing source, twoleads connecting the energizing source to said heating element, two inductors each having two terminals and each con nected in series With oneof said leads, two con tact plates, each contact plate being injuxtaposition to one of said inductors, two isolating resistors with arelatively high impedance to the heating energy in comparison with theimpedance of said inductors, each of said resistors connected betweenone terminal of an inductor and its associated said contact plate, and aplurality of push buttons, each of said push buttons having a pair ofshorting contacts attached to the end thereof adapted to makesimultaneous contact between a predetermined turn of each of saidinductors and its associated contact plate when said push button isdepressed.

8. Apparatus for varying inductance in series with a heating elementlead of an electron discharge device comprising a heating element forsaid electron discharge device, a heating element energizing source, twoleads connecting the energizing source to said heating element, aninductor having two terminals connected in series with each of saidleads, two contact plates, each con tact plate being in juxtaposition toone of said inductors, two isolating resistors With a relatively highimpedance to the heating energy in comparison with the impedance of.said inductors, each of said resistors connected between one terminal ofan inductor and its associated said con tact plate, and a plurality ofpush buttons, each of said push buttons having a pair of shortingcontacts attached to the end thereof adapted to make simultaneouscontact between a predetermined turn of each of said inductors and itsassociated contact plate when said push button is depressed, and meansfor supplying radio frequency energy to the other terminals of saidinductors.

9. In a device for supplying heating current to the filament of anelectron discharge device, the combination comprising a heating elementfor said electron discharge device, a filament transformer having aprimary and secondary winding, a source of alternating current connectedto the primary winding of said transf0rmer, two leads connecting thesecondary winding of said transformer to said filament, an inductorhaving two terminals connected in series with one of said leads, acontact plate in juxtaposition to said inductor, an isolating resistorwith a relatively high impedance to the heating energy in comparisonwith the impedance of said inductor, said resistor connected between oneterminal of said inductor and said contact plate, and a plurality ofpush buttons, each of said push buttons having a shorting contactattached to the end thereof adapted to make simultaneous contact withthe contact plate and a predetermined turn of said inductor when thepush button corresponding to said turn is depressed.

10. In a device for supplying heating current to the filament of anelectron discharge device, the combination comprising a heating elementfor said electron discharge device, a filament transformer having aprimary and secondary winding,

a source of alternating current connected to the primary winding of saidtransformer, two leads conneeting the secondary winding of saidtransformer to said filament, two inductors each having two terminalsand each connected in series with one of said leads, two contact plates,each contact plate beingin juxtaposition to one of said inductors, twoisolating resistors with a relatively high impedance to the heatingenergy in comparison with the impedance of said inductors, each of saidresistors connected between one terminal of an inductor and itsassociated said con tact plate, and a plurality of push buttons, each ofsaid push buttons having a pair of shorting contacts attached to the endthereof adapted to make simultaneous contact between a predeterminedturn of each of said inductors and its associated contact pla-te whensaid push button is depressed.

11. In a device for supplying heating current to the filament of anelectron discharge device, the combination comprising a heating elementfor said electron discharge device, a filament transformer having aprimary and secondary winding, a source of alternating current connected to the primary winding of said transformer, two leads connectingthe secondary winding of said transformer to said filament, twoinductors each having two terminals and each con nected in series Withone of said leads, two con tact plates, each contact plate being injuxtaposition to one of said indutors, two isolating resistors with arelatively high impedance to the heating energy in comparison with theimpedance of said inductors, each of said resistors connected betweenone terminal of an inductor and its associated said contact plate, and aplurality of push buttons, each of said push buttons having a pair ofshorting contacts attached to the end thereof adapted to makesimultaneous contact between a predetermihed turn of each of saidinductors and its associated contact plate when said push button isdepressed, said inductors forming part of a parallel tuned cathodecircuit for said electron discharge device, said tuned cathode circuitincluding a capacitor arrangement in shunt to said induct0rs.

CHARLES JAMES STARNER.

N o references cited.

